Amplifying devices for telephone hand-sets



June 16, 1959 C. H. MILLER AMPLIFYING DEVICES FOR TELEPHONE HAND-SETS Filed Sept. 12, 1956 SNN i between'a inirophone `2 and the mout jnite i This invention relates' "to amplifying devicesl for use 15 Withtelephone hand-sets, such devices Vvbeingofthe type' comprising a first signal channel including a microphone,"

a first amplifying means, `a first loudspeaker for operation" by the output from the amplifying means, said `first loudspeakeribeing` arranged to supply audible'signals` t'oV 20 the 'mouthpiece 'of a conventionaltelephone hand-set;

a second" signal c'hannel including means responsive to' sig'nal's derivable from'the ear-piece ofthe hand-set, a secondiamplifying means for such signals, and a second loudspeaker for operation by 'the output of said' second 25 amplifying means, and switching means for maintaining eitherone of .the ."channels ineffective as regards transmissidnjof signals applied thereto. A

According to "the "present invention there is provided an Aamplifying -device of the type referred to, wherein 3UR said switching means includes an electronic "oscillator" which-is arranged `to producean oscillating output in responseto a control voltage input in" such away that, when said voltage exceeds a predetermined valuean oscil-` lating output is at once obtained and continues until the 35 instant when said ,voltage no longer exceeds the predetermineclvalu'e, andwherein one o f said channels hasan elec- 'j tronic'. switch device therein which renders such channel j ineffective -for transmission of voicesignalsito itsloud#` spaker` when no incoming signals are being applied toy40 either channel,l whereby `the other channel is normally operative, there being a connection between 'saidonechan-- nel and thevoltage input to said electronic oscillator andI between the `outputof the electronicfoscilla'tor and said electronic switch device, the' connections and arrangement` 45 being such that when voice signals Aare being effectively' transmitted in said other channel the electronic oscillator is ina condition vsuch thatthe saidelectronic switch device makes said one channel ineffectivewhilstwhen such voice signals ,cease and when voice signalsjare lappliedmto said 50i one channel," the control voltageinput is derived therefrom for said electronic oscillator which latter atoncecauses said electronic switch device toassume an operative position where said one channel is effective for transmission of speech signals to its loudspeal e12- I ,For aabetter understanding of theinvention'and to show how the "same, may be carried into"effeet`,rrefrenc will novi` beto the'accompanying drawihgwhicli is a cir-cAA cuitdiagraiiiof Aan amplifying device forbsie `with` telei" phoheaHdfSefS a and'is connectedtoreceive input signals'from'themicrof` -ph'on'e"2. Output signals `from'tl'iefirststage 3 are fed i to the control grid d of a pentode valve. 5 lforming theO scoldjstagc of 'the three stage amplifier. x An on-'potf 10 Switch @A iSPfQYided forth@ :litt .Channslsaraef. tance 6 is provided in the signal path from the "first stage 3 States Patenthtice 2,891,116 Patented dune i 1 6, 959-1 a t the control grid'4 to allow a D.C`.-gain controllingwbias "to be appliedvia thefresistance 4A tofthe control grid 4 of the valve 5.:

Outpt from valve is taken from the anode 7 thereof;-4 and is fed` via a capacitance 8, and an electronicswitoh-f ingV arrangement 9 to the inputof the nalstage 1d of Athe three stage amplifier. The final stage 10 acts as an ampli-ffier'for a loudspeaker 11, the latter-being'- arranged-@for feeding energy t'o the mouth-piece `ofthe telephone hand` set;

A second-audio-'frequency amplifying channel '-12 is used for" detecting andamplifying audio-signalsfreceived -aty the ear-piece of the handset (not shown)l andfor re-A producing the` received signals :in a loudspeaker. c

The channel`12 includes an inductive pick-up 13,-Which` is arranged to receive, as input,output1signals magnetical ly induced from the ear-piece of the handset. Voltage outputfrom the pick-up *13 is applied to a rst staged@-- of a multi-stageamplier'provided inthe channel12. The stage 14' 'appliesanV amplified signal via an adjustable arripliei"` gain control arrangement'll to the control grid l@ of a 'valve' 17, the `latter forming theV second amplier stage `oftheV amplifying channel 12. The gain control ar-` rangement 15' includes a resistance l connected between the output ofstage i4 and earth `together with `a variable' tap 19 for taking off the desired signal strength, the latter beingefedyia a capacitance 2th to said `control grid 16J Amplified output from the valve17 is taken from the anode ,21;A thereof and is fed `to the input of thenal amplifying stage: of the amplifying channel 12-via-a` capacitance 22, a resistance Ztand an electronic switching arrangement 24. Output from the stage-25 isiapplied asinput'to a loudspeakerzd. The loudspeaker `26 repro duces audibly with amplifica-tion the magneticallyv induced signals received at the ear-piece of the hand-set. Output from the stage 25 is also' fed via a capacitance- 41A to-an input 37 `of a switching oscillator 38.l

In 'order fto take'into 'account the 4effects of ambient noise' aroundthe amplifying device athird amplifying channel 27 is included in the amplifying device. The channel 27' includes a microphone 2S `the function -of which is' to detect surroundinga noise signals, and a two-stage variable' gain amplifier29. Output from the amplifier- 29'is fed1 viaa capacitance Si), the `anode of a-diode rectifier"`31` whosecathode 31A is eartlied,` and a resistance 32"'to" a sainte.

:Output signals from the third stage 2S of theanplifyin ichannelJlZ are `alsoappliedvia a capacitance 41,a recti tier `4t2 and resistance '43 to the `point AQ The pir 'of :series rcoimected resistances 33, `34 is connect'edbetwe" the' point A andi earth, forming a return path' cinmn to both diodesl and 42l Asmoothing 'capacitance 35" is also connected frompoint Afto earth; By'thismeans the output signalshwhen present, of thetwo amplifying channels Hyland Z7 may be combined Hand -appliedas 'a smooth negative D.C. 4bias via-resistance 36to" the Ainput 37 of the switching'oscillatrSS c c Output from thevalve 5 of amplifying channel" 1 .isi'also fed toth'einput 37 4ofthe switching osillator' 38,.i :a direct A.C. connection being made from the"anodef7 of thevalve 5 via -a capacitance 401 `Switchingoscillator-'aS-is an electronic oscillatorbiase'd in thehun-oscillating condition in such?afmanner'thatwhen the peaks of waveform of the-alternating ipuffro'rii" the anode 7r of the' valve -5` exceeds apredeternin'ed Jvalue the oscillator at "once producesoscillations for' s lori'g' Las the 'predeterminedvalue is exceeded. nf`tlie"pre`sent` instance the switching oscillator` 38 may consist of' a` conventional multi-vibrator, `an initial Jpr'ede't'ermin'ed "op-f :erhatingpoint` being controlled by a positive potential" :applied iriV sries with the input37.4 a

Tkfe" eit 'et th" negative D.C. bias" applied to the" input 37 of the oscillator 38 from point A via the resistance 36 is to raise the level of the operating point at which the oscillator can be caused to oscillate by the alternating signal from valve 5. To prevent excitation of the oscillator 38 by very large alternating signals from the anode 7 of valve 5, a fraction of the negative bias at the point A is fed from the junction of the resistances 33and 34 via resistance 4A to the control grid of valve 5, in order to reduce the amplification of valve 5. Capacitance 4B serves as an additional smoothing arrangement.

' An automatic volume control line 44 for the second amplifying channel is connected between the point A and the input of the rst stage 14 of the amplifying channel 12. AThe line 44 includes a resistance 45 and a diode 46. A positive bias from a source of positive potential (not shown) is applied via a resistance 45A to the anode of the diode 46. The diode 46 serves to delay the action of the automatic Volume control. The cathode of the diode 46 is earthed, while the anode thereof is connected via a resistance 46A and the pick-up inductor 13 to the control grid of the valve of the rst stage 14.

The switching oscillator has an output 47 which is connected to a point B, via a diode rectiiier 48 and a delay resistance-capacitance-network 49. The signal which appears at B and which emanates from the switching oscillator is used for controlling the operation of the electronic switching arrangements 9 and 24.

The switching arrangement 9, provided in the signal path of the first amplifying channel 1 includes, after the capacitance 8, a pair of series connected resistances 50 and 51. The anode 54 of a diode 52 is connected to the junction of the resistances 50 and 51. The cathode of the diode 52 is earthed. In addition the anode 54 is connected via a resistance 55 to the point B. That end of the resistance 51 which is remote from said junction is connected to the anode 56 of a diode 53 whose cathode is earthed, and to the point B by way of a resistance 57.

The point B is also connected to the cathode 58 of a diode valve 59, which latter forms part of the switching arrangement 24. The anode 60 of the diode 59 is connected to the anode 21 of the valve 17 via the resistance 23 and capacitance 22, and also the control grid 16 of the valve 17, via a resistance 61 and a resistance 62. The junction of the resistances 61 and 62 is connected to a positive source of potential (not shown) through a resistance 63. The cathode of the valve 17 is connected to this source by Way of a resistance 64 whilst a capacitance 65 acts. as a decoupling capacitance for the control grid 16.

A further source of positive potential (not shown) is connected to the point B via a resistance 66, to provide positive biasing potentials for the anodes of the diodes 48, 52 and 53 and the cathode 58 of the diode 59.

When there are no voice signals in either of the channels 1 or 12 the initial switching conditions of the switching arrangements 9 and 24 are such that the.

amplifying channel 1 is normally suppressed, i. e. ineffective, Whilst the amplifying channel 12 is normally actively effective.

To achieve the suppressed state in the first amplifying channel 1, the D.C. potential appearing at the anode S4 of the diode 52 causes it to conduct` so that any low level signals, appearing in the signal path of the first channel, but of insuicient amplitude to trigger the oscillator 38, are fed through the diode 52 to earth. To ensure that any residual signals, which may by-pass the diode 52, do not reach the nal amplifier stage V10, `the second diode 53 acts in a similar manner to the diode 52. That is to say the diode 53 oiers a low resistance path to signals in the first channel 1. Thus as a result of the dual effects of the diodesl 52, and 53 no signals will reach the loudspeaker 11. The biasing potentials on the diodes 52 and 53 are derived from the positive source, connected to the resistance 66, the potentials being fed to the anodes 54 and 56 via the resistances 55 and 57 respectively.

The second amplifying channel 12, as previously mentioned, is normally receptive to incoming signals at the ear-piece of the hand-set. Any such incoming signals are detected by the inductor pick-up 13, and are fed to the iirst amplifier stage 14, wherein they are ampli- Alied. The output from the rst stage 14 is applied to the control grid 16 of the valve 17 via the amplifier gain control device 15. Output from the valve 17 is fed by way of the switching arrangement 24 to the last stage 25 of the amplifying channel 12, and thence to the loudspeaker 26, the biasing potentials applied to the cathode 58 and anode 60 of the diode 59 of the switching arrangement 24 being such that the diode 59 is normally non-conductive.

Upon the receipt of an incoming signal at the earpiece of the hand-set the amplied version of this appearing at the output of the iinal amplifier stage 25 of the amplifying channel 12 is utilised to modify the switching conditions so that the amplifying channel 1 is suppressed even against large acoustic inputs to the microphone 1. The amplified output signal is fed from the nal amplifier stage 25 to the anode of the diode 42 via capacitance 41. The diode rectiiies this A.C. signal and the resulting negative D.C. potential is fed via the resistance 43 to the point A, charging the capacitance put from reaching the stage 10 and the loudspeaker 11.

When the inductor pick-up 13 ceases to pick up signals from said ear-piece, the negative biasing potential arising from the nal stage 25 and which is applied to the input 37 of the oscillator 38, disappears. The switching oscillator 38 may then be triggered by any voice or noise sigv ceed the predetermined operating point.

nal received by the microphone 2 and which, after amplification is fed from the anode 7 of the valve S to the input 37 at sufficient amplitude for the signal peaks to ex- The output of the oscillator, thus caused to function, is rectified by the diode 48, and appears at point B as a smoothed negative bias. The amplitude of this negative bias is large enough to overcome completely the elect of the standing positive bias at the point B, so that a negative potential is fed via.

resistances 55 and 57 to the anodes 54 and 56 of the diodes 52 and 53 rendering these non-conducting. Voice or noise signals at the anode 7 of valve 5 then pass via capacitance 8, resistances 50 and 51 -to valve 10 and the loudspeaker 11, which latter transmits them into the mouth-piece of the telephone.

At the same time the negative bias from the point B is applied to the cathode 58 of the diode 59 causing this to conduct and bypass any subsequent signal owing through resistance 23 to earth via capacitance 58A. The negative bias from point B also flows via diode 59, resistances 61 and 62 to the control grid 16 of valve 21, thereby reducing its gain or cutting -it oi.

As stated either a voice or noise signal received by the microphone 2 of channel 1 Will cause this latter to be- To prevent this, a noise microphone 28 is provided si multaneouslyto detect and pick up any `general surrounding noise signal and these latter mentioned noise signals are amplitied in the amplilier 29;

t The A.C. output froml the amplifier 29 is fed by way of the capacitance 30 to the anode of the diode 31 which latter produces a negative D.C. potential representative of the output from the amplilier 29. This latter mentioned negative D.C. signal is applied via the resistance 32, the point A, and resistance 36 to the input 37 of the switching oscillator 38. The effect on the oscillator 38 of this latter mentioned negative D.C. potential is arranged just to counteract the effects produced by the A.C. potential representative of surrounding noise detected at the microphoneY 2, and so prevent operation of the oscillator 38by these said `A.C. noise potentials. The mentioned negative D.C. potentials derived from the output of amplifier 29 are small and do not prevent subsequent operation of the oscillator 38 by normal voice signals.

Recapitulating, it will be noted that there are simulaneously applied to the switching oscillator 38, three potentials, namely an A.C. potential applied to the input 37, which is characteristic of surrounding noise signals picked up by the microphone 2 in the rst amplifying channel 1, a negative D.C. potential, fed to the input 37, characteristic of surrounding noise signals detected by the microphone 28 in the third amplifying channel 27, and negative D.C. potential, applied to the input 37, derived from the incoming speech signals received by the inductor piek-up 13. The D.C. potential derived from the rectifier 42 is of suiicient magnitude to prevent the users voice, when speaking into the microphone 2, from operating the switching oscillator 38. Thus it will be seen that so long as signals are being received at the inductor pick-up 13, the user cannot trigger the oscillator 38. Consequently the switching oscillator 38 is maintained in its initial state, in which no output signal is obtainable therefrom.

As a further safeguard to prevent the triggering of the oscillator whilst incoming signals are appearing at the hand-set the gain of the valve is reduced by a further D.C. potential obtained from point A by reason of the charging of the capacitance 35. This latter mentioned D.C. potential is a fraction of that fed to input 37, and is applied to the control grid 4, via the resistance 4A. The gain of the valve 5 is progressively reduced in relation to the increase of the negative potential on capacitance 35, which latter is charged in relation to the output from the linal stage 25. This reduction in gain ensures that the positive peaks of any noise signals picked up by the microphone 2 cannot overcome the effects of the negative D.C. potentials applied to input 37 of the oscillator 38, thereby to trigger the latter.

The automatic volume control circuit for the second amplifying channel operates as follows: The negative D.C. potential produced at A, as a result of the A.C. signal fed from amplier stage 25, is applied along the line 44, via the resistance 45, to the anode of the diode 46. The biasing potentials of the diode 46 are such that it normally conducts, that is to say there is a direct conductive path therethrough to earth. However, when the negative D.C. potential produced at A attains a predetermined value, thediode 46 ceases to conduct, hence any negative bias, exceeding this predetermined value, is applied to the input side of the amplifier stage 14, by Way of the resistance 46A and the inductor pick-up 13.

Since a nite time period is required for the D.C. potential resulting from the A.C. signals obtained from incoming sound signals in the second channel, to attain said predetermined value, the action of the automatic volume control circuit is delayed inasmuch as it is only operative whenever the D.C. voltage exceeds said predetermined value.

When the inductor pick-up ceases to pick up signals from said ear-pieces the negative biasing potential, which arises from the final stage 25 and which is applied to the input 37 of the oscillator 38, disappears. When, however, surrounding noise alone is picked up by the microphone 2, an A.C. signal representative of this noise would normally be fed from the valve 5 to the input 37 of the oscillator 38, resulting in the opening for transmissiori of the channel 1 and the closing for transmission of the channel 12. To prevent this occurring a rectified signal derived from the noise detecting microphone 28 is fed to the input 37 of the oscillator 38. The effect of this rectified signal is to bias the oscillator 38 so that reception by it of the noise representative A.C. signal from the valve 5 does not actuate the oscillator 38.

The removal of the negative D.C. bias also simultaneously removes that fraction thereof which is used to reduce the gain of valve 5, whereupon the gain of this valve 5 is restored to its usual value.

When the user speaks into the microphone 2, the users voice signals are amplified in the Valve 5 operating at full gain, thereby producing an A.C. voice signal at the output side of thevalve 5. The A.C. voice signal is applied to the input 37 of the switching oscillator 38. This oscillator is so arranged that during the receipt of the positive peaks of the A.C. voice signal it produces oscillations in the output 47 thereof. These oscillations are rectiiied by the diode 48, to produce a D.C. output characteristic of the oscillations, the latter mentioned D.C. output being fed to the point B via the resistance-capacitancenetwork`49. This D.C. output is of negative polarity and, in consequence, changes the polarity of the potential at the point B from its initial positive value to a negative value.

As soon as the positive potential at the point B is exceeded by the negative D.C. potential from the diode 48, the diodes 52 and 55 become non-conductive, and no longer shunt signals incoming via resistances 50 and S1. Whereupon the users speech signals are fed to the amplifier stage 10, and thence to the loundspeaker 11, which latter feeds sound energy to the mouth-piece of the handset.

The negative D.C. potential at the point B is also applied to the cathode 58 of the diode 59 in the switching arrangement 24. This negative D.C. potential reverses the polarity of the cathode 58 and causes the diode 59 to conduct. As soon as the diode 59 conducts, any incoming signals which may now be appearing at the output of the valve 17 are shunted to earth via the condenser 58A. The negative D.C. potential is also conducted via resistances 61 and 62 to the grid 16 of the valve 17. This D.C. potential biases the grid 16 of the valve 17, thereby to cut off the valve 17. The combined effects of the conductivity of diode 59 and the cutting o" of valve 17 renders the second amplifying channel non-conductive.

I claim:

l. An amplifying device for use with a telephone handset comprising a first signal channel, a microphone connected to said iirst channel, a irst amplier included in said rst channel, an output of said first amplifier, a loudspeaker connected to said output of said rst ampliiier for operation thereby and for feeding audible signals to a mouthpiece of a telephone handset, a second signal channel, means responsive to signals derivable from an earpiece of said handset and connected to said second channel, a second amplifier connected in said second channel, an output of said second amplifier, a second loudspeaker connected to said output for operation thereby, an electronic switching device included in one of said two channels, an electronic oscillator, an oscillator input, means for deriving a control voltage from said one channel and connected to said oscillator input for applying thereto said control voltage, an oscillator output at which an oscillating voltage appears when said control voltage exceeds a pre-determined value, means responsively coupling together said two channels for controlling the magnitude of said control voltage and means for responsively connecting said oscillator output to said electronic switching device for causing said switching device to block said one channel to the transmission of signals when voice signals are being transmitted in said other channel .andalso 'whenzn'orvoicez'signals are being transmitted .in -ieither channel, .and for causing -said switching device :to `op'en :said one channel 5to `the transmission of voice fsignals when `no voice :signals are transmitted yin said other channel but when voice signals transmitted in said .one channel result in said control voltage reaching said pre-determined value.

2. 4'An amplifying device accordingto claim l, wherein said .means for responsively coupling together vsaid channels comprises means for deriving `a control .parameter from -said other channel, Which parameter is responsive to the transmission .of voice signalsin said `other channel, and means for applying thefcontrol parameter so derived to said one channel so as lto 'control the .magnitude -of said control voltage.

3. .-An'amplifying device faccordingrto claim l, wherein said :means for responsively coupling .together .said channels .comprises means `for deriving a control parameter from lsaid other-channel, =which parameter is responsive tothe transmission of voice-signals in said other channel, and means for applying the parameter so derived to the input of -said oscillator so as to control "the magnitude of the vcontrolwolltage `applied to 'said oscillator.

4. An amplifying device according Ito yclaim l, and

furthermore comprising a further 1electronic switching 25 device'included in said other channel, and further means for Iresponsively .connecting said oscillator output to said said means for applying the control pa-ram'eter soderivedlto :said ione Lchannel is connected =to fsaidfrst amplierfso'as tovarfyTthe-gain o'f said-'rstampliiier. Y

,6. amplifying friet/'ice according 4to claim -1, and furthermore comprising ydelay means eoupled to :said

oscillator output wfor delaying the .application fof said. oscillating Voltage .to .said'electronic switching device so,v

as to avoid -the production :in `said channels of audible noises Idue fte switching transients. p i

F7. .-An amplifying :device according to claim .1, and furthermore comprising means -forderiving an electrical parameter responsiveto -theintensitypf surrounding Jnoise signals and means for applying theparameter vso derived to :the input of 'said .oscillator so as to ycompensate for the portion -.of said control voltage arising -from said .surrounding'noise signals as detected .by` said-microphone. l

References-Cited in the'ile of this patent OTHER REFERENCES Varley July v18, 1'93'9. Di Toro et-.al vJan. 11, '19*55` 

